Follow these steps if you get this message on Youtube "an error has occured-please try again later":
1) Disable all RealPlayer Plugins or Extensions on FireFox then close Firefox.
2) Launch RealPlayer and click on the RealPlayer icon at the top left.
3) Click Preferences
4) In the Category panel click Download and Recording.
5) Uncheck (remove check mark) from Web Download and Recording and click OK.
6) Start Firefox and try to play the Youtube videos.